Create a PDF file from resized images:
for file in *.png; do convert $file -resize 600 "imgtemp-$file"; done && convert imgtemp-*.png all_pngs.pdf && rm imgtemp-*.png
Legal stuff about this site and your private life:
If you're not OK with this, move your way.
Create a PDF file from resized images:
for file in *.png; do convert $file -resize 600 "imgtemp-$file"; done && convert imgtemp-*.png all_pngs.pdf && rm imgtemp-*.png